1. Field of the Invention
The present invention relates to a gas sensor for measuring gas components such as NO, NO2, SO2, CO2, and H2O contained, for example, in atmospheric air and exhaust gas discharged from vehicles or automobiles. In particular, the present invention relates to a gas sensor having a protective cover which is arranged to surround a sensor element.
2. Description of the Related Art
At present, a variety of gas sensors have been suggested and practically used, including, for example, oxygen sensors and NOx sensors based on the use of oxygen ion conductors (see Japanese Laid-Open Patent Publication No. 8-271476), HC sensors (see Japanese Laid-Open Patent Publication No. 8-247995), hydrogen sensors based on the use of proton ion conductors, H2O sensors, and oxygen sensors and various gas sensors based on the use of oxide conductors such as SnO2 and TiO2.
Among these gas sensors, the oxygen sensor based on the use of ZrO2 and the oxygen sensor based on the use of TiO2 keep stable performance even in an environment of exhaust gas discharged from the automobile. Therefore, they are widely used to control the oxygen concentration in the exhaust gas of the automobile and control A/F. The NOx sensor based on the use of ZrO2 is also at a stage of practical use to control NOx for the automobile.
Those known as the oxygen sensor to be attached to an exhaust tube of an internal combustion engine include those having a protective cover which is intended to obtain a uniform flow of exhaust gas around a sensor element as described in Japanese Laid-Open Patent Publication No. 1-169350, and those having a protective cover which is intended to avoid any adhesion of condensed water (so-called water splash) caused when an engine is started. An oxygen sensor, which is attached with a protective cover having a double structure, is known as described in U.S. Pat. Nos. 4,597,850 and 4,683,049.
As for those having the conventional protective covers as described above, it is feared that the response performance may be slow in the case of the gas sensor provided with the protective cover which has the resistance to water scattering so that the water splash is prevented. Accordingly, a protective cover having the double structure has been suggested, wherein the response performance is improved such that an inner gas-introducing hole of an inner protective cover disposed adjacently to a sensor element is provided opposingly to the sensor element (see Japanese Patent No. 2641346).
However, the illustrative conventional structure described above is designed assuming that the sensor is installed on the upstream side of a catalyst. It has been revealed that when the sensor is used while it is installed on the downstream side of the catalyst, it causes a problem concerning the water scattering resistance (performance to avoid adhesion of condensed water caused when the engine is started).
As described above, when the structure of the protective cover is designed such that the input amount of the measurement gas is increased in order to obtain quick response, the condensed water, which is produced when the engine is started, tends to make invasion as well. In other words, the effect to prevent the sensor element from water splash is related to be contrary to the improvement in response performance. It is difficult to simultaneously satisfy these effects.
On the other hand, a cycle is assumed, in which the air-fuel ratio of exhaust gas is changed from the lean (atmosphere of excessive oxygen) to the theoretical air-fuel ratio or the rich, and the air-fuel ratio is returned again to the lean at a stage at which NOx is completely released from a catalyst, in order to reduce NOx occluded by the catalyst, when a NOx sensor having a function of oxygen sensor is attached on the downstream side of the NOx-absorbing catalyst. In such a case, if the complete release of NOx from the catalyst can be detected at an early stage, it is possible to decrease the unburned gas leaked from the catalyst.
The present invention has been made taking such problems into consideration, an object of which is to provide a gas sensor having a novel protective cover provided with resistance to water scattering, in which the gas diffusion rate-limiting can be decreased to be as small as possible, and the protective cover simultaneously satisfies both of high water scattering resistance and quick response performance.
The present invention lies in a gas sensor comprising a sensor element for measuring a predetermined gas component contained in an introduced measurement gas, and a protective cover arranged to surround the sensor element; wherein the protective cover includes an inner protective cover for covering at least a forward end portion of the sensor element; an outer protective cover for covering the inner protective cover; and an intermediate protective cover installed between the inner protective cover and the outer protective cover.
That is, the protective cover, which is used for the gas sensor according to the present invention, has a triple structure in which the intermediate protective cover is provided in the cover having a double structure composed of the inner protective cover and the outer protective cover. Therefore, it is possible to effectively avoid the adhesion of condensed water (so-called water splash) which would be otherwise caused when the engine is started. Further, it is possible to decrease the diffusion rate-limiting of the measurement gas as small as possible. Thus, it is possible to obtain quick response performance.
Accordingly, for example, when the gas sensor is attached on the downstream side of a NOx-absorbing catalyst to assume a cycle in which the air-fuel ratio of exhaust gas is changed from the lean to the theoretical air-fuel ratio or the rich, and the air-fuel ratio is returned again to the lean at a stage at which NOx is completely released from the catalyst, in order to reduce NOx occluded by the catalyst, then it is possible to detect the complete release of NOx from the catalyst at an early stage, and it is possible to decrease the unburned gas leaked from the catalyst.
In the gas sensor constructed as described above, the inner protective cover is formed to have a bottom-equipped cylindrical configuration with an inner gas-introducing hole which is formed at a position opposed to the sensor element and with an inner gas discharge hole which is formed at a bottom portion; the outer protective cover is formed to have a bottom-equipped cylindrical configuration with an outer gas-introducing hole which is disposed at a position not opposed to the inner gas-introducing hole of the inner protective cover; and the intermediate protective cover has an intermediate gas-introducing hole which is disposed at a position not opposed to the inner gas-introducing hole of the inner protective cover and the outer gas-introducing hole of the outer protective cover.
Accordingly, the water is introduced from the outer gas-introducing hole of the outer protective cover, most of which collides with the side surface of the intermediate protective cover and the side surface of the inner protective cover. The gas sensor is prevented from invasion of water into the inside. That is, the sensor element is prevented from invasion of water. The water is discharged to the outside without any delay through the outer gas-introducing hole of the outer protective cover.
Further, the measurement gas, which is introduced from the outer gas-introducing hole of the outer protective cover, passes through the intermediate gas-introducing hole of the intermediate protective cover and the inner gas-introducing hole of the inner protective cover, and it arrives at the sensor element. After that, the measurement gas is discharged through the gas discharge hole formed at the bottom portion of the inner protective cover and the outer gas-introducing hole of the outer protective cover. It is noted that the negative pressure is generated in the vicinity of the gas discharge hole disposed at the bottom of the inner protective cover. Therefore, the measurement gas flows quickly through the flow passage as described above. Accordingly, the diffusion rate-limiting of the measurement gas is decreased, and it is possible to obtain quick response performance.
It is preferable for the gas sensor constructed as described above that a total of a most adjacent spacing distance in an axial direction between the outer gas-introducing hole and the intermediate gas-introducing hole and a most adjacent spacing distance in the axial direction between the intermediate gas-introducing hole and the inner gas-introducing hole is at least not less than 10 mm. More preferably, the total is not less than 18 mm.
It is preferable that gaps for avoiding accumulation of water due to boundary tension are provided between the outer protective cover and the intermediate protective cover in a radial direction and between the outer protective cover and the inner protective cover in the radial direction.
It is also preferable for the gas sensor constructed as described above that the intermediate protective cover is formed to have a cylindrical configuration with an opening which is formed at a front portion thereof for inserting the inner protective cover thereinto and with a flange which is disposed at a rear portion thereof for making abutment against an inner wall of the outer protective cover. It is also preferable that the intermediate protective cover is formed to have a bottom-equipped cylindrical configuration with an intermediate gas discharge hole which is formed at a bottom portion thereof.
It is also preferable that a part of the outer gas-introducing hole of the outer protective cover is provided at a side surface portion of the outer protective cover between the inner gas discharge hole or the intermediate gas discharge hole and a bottom of the outer protective cover.
It is also preferable that the intermediate gas-introducing hole is formed at the flange of the intermediate protective cover.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.